The present invention relates to circuit protection and more particularly to resettable circuit protection.
Automotive fuses, such as blade type fuses are known in the art. Modern electrical blade fuses have been manufactured by Littelfuse, Inc., the assignee of the present invention. Blade fuses protect electrical automotive circuits from current overloads. The protection results from an opening of a fuse element located within the fuse, which stops the flow of current to the circuit protected by the fuse. Upon a current overload of a certain magnitude and over a predetermined length of time the fuse element or link breaks or opens.
Automobile manufacturers are constantly adding more electrical devices and accessories to automobiles. Consequently, automobile circuits having increasingly higher operating voltages, e.g., 60 volts to over 700 volts, are being contemplated and implemented. Higher ratings require more robust conductive elements and more insulation. The trend towards lower cost therefore competes against the trend towards higher capacity.
Known blade fuses employ: (i) an insulating housing; (ii) conductive male terminals that fit into female terminals extending from the automobile's fuse block; and (iii) a fuse element connecting the male terminals. The male terminals have typically extended below the insulating housing. When installed in the fuse block, the housing of the fuse sits above the female terminals. The housing in such configuration and placement provides a convenient apparatus or place of the fuse to be grasped and pulled or pushed to remove or replace the fuse, respectively.
One concern facing all blade-type fuses arises when the fuse element opens. The opening of the fuse element coincides with a release of energy, particularly heat energy. Fuses are thermally activated devices, which rely on heat generated at the element to open the circuit. Particularly at low-overloads (e.g., 110% to 200% of the rated current of the fuse), a significant amount of heat is created, which can interfere with the immediate surroundings of the fuse. It is therefore desirable to mitigate or eliminate potentially harmful energy dissipation effects caused when blade fuses open.
Another issue regarding blade fuses is that they are not resettable. A blade fuse must be replaced after it has opened. Most blade fuses has diagnostic or integrity contacts so that each fuse can be tested without having to pull the fuse from the fuse block. Nevertheless, integrity testing various blade fuses to determine which one has opened can be time consuming. When it is found the fuse has no ability to show why or when it opened. Moreover, until it is replaced the load that is being protected may be partially or totally impaired. It is therefore also desirable to provide a resettable over-current protection device for automobiles.
Fuses, such as blade fuses, are selected for both their current rating as and their I2t value (or let-through energy). In many applications surges occur during the normal operation of equipment, and the fuse must allow for such operation. For overloads lasting longer than a few minutes, the fuse in many cases is selected primarily on the basis of its continuous current rating, wherein such current rating is greater than the extended overload current.
For infrequent normal overloads, fuse selection can be made on the basis of an overload curve (I2t curve), wherein the fuse is selected based on, e.g., 75% of its published time−current curve. For repetitive overloads, such as a motor drive or soft starter, fuse selection can be made on the basis of the overload curve (I2t curve), wherein the fuse is selected based on, e.g., 60% of its published time−current curve. Indeed, fuses typically have two I2t ratings; a clearing rating and a melting rating. The clearing I2T rating is the total I2t passed by the fuse as the fuse clears a fault. The Melting I2T rating is the minimum I2t required to melt the fuse element. It is therefore further desirable to provide an improved over-current protection device for automobiles, which has known and controllable I2t ratings.